1. Field of the Invention
The present invention relates generally to the testing of telephone lines and systems, and particularly to telephone test sets for accessing and testing subscriber lines. More particularly still, it relates to a telephone test set capable of testing subscriber lines emanating from different switching centres which use the following modes of signalling: dial pulse; DTMF or tone signalling; and digital command signalling on subscriber lines associated, for example, with the now widely used Northern Telecom DMS-100 switching system.
2. Prior Art of the Invention
Prior art telephone line testing devices may be used on telephone lines by simply connecting them to the tip and ring leads. U.S. Pat. No. 4,194,098 granted Mar. 18, 1980 to Carlson and U.S. Pat. No. 4,682,346 granted Jul. 21, 1987 to Faith et al provides such test sets which are capable of operating on telephone lines which use either of dial pulse or DTMF signalling both of these United States Patents are relevant prior art as far as the dial pulse and DTMF signalling functions are concerned. Both United States Patents are incorporated herein by reference.
On telephone lines switched using a digital telephone switch such as Northern Telecom's DMS-100 such prior art devices cannot be used. A factor in such incompatibility is that although the prior art devices have included universal signalling test systems (U.S. Pat. No. 4,165,450) and occasionally had selective D.C. impedances of 600 ohms and 900 ohms (U.S. Pat. Nos. 4,843,620 and 4,912,755) they have generally had impedances in the range of 70 to 300 ohms. A digital telephone line requires the testing device D.C. impedance to be in the range of 1000 to 2000 ohms. Overcoming such incompatibility would simplify the number and type of devices that would be needed to test telephone lines in various telephone line systems.
Prior art devices for testing the circuitry of telephone lines have included memory dialers. They have also included batteries to provide power to the memory circuitry when the device is not connected to the tip line and ring line as a source of power. Batteries have various replacement and maintenance problems associated with them. Such problems could be overcome by using a different power source for the memory circuitry.
Moreover, prior art devices have included a shunt circuit to limit the current applied to the device. Usually, such shunt circuits have comprised sensing circuits and a silicon controlled rectifier to shunt current from an overcurrent condition between the tip line and ring line.
Additionally, prior art devices have included light emitting diodes in series with voice receiving and transmitting units. Alternatively, the diodes were installed across the tip line and ring line with a switch to momentarily test the polarity of the voltage on the tip and ring lines.
Finally, prior art devices included circuitry producing a short between the tip and ring lines to keep large currents out of the device. Such circuitry could result, if the large current was from an unfused source or a source with a high current fuse (such as 15 ampheres or higher), in damage to the devices and injury to a person using them. Such damage and injury may be avoided by opening the tip and ring lines, not allowing current to flow.